Transformable stainless steel



United Stat O i I 2,967,770 I TRANSFORMABI JE srAnsLEss STEEL I Albert Hall, David C. Ludwigson, and Donald B. Roach, Columbus, and Virgil W. Whitmer, Canton, Ohio, assignors, by mesne assignments, to Republic Steel Corporation, Cleveland, Ohio, a corporation of New Jersey No Drawing. Filed May 29, 1959, Ser. No. 816,681

6 Claims. (Cl. 75-125) invention relates to the manufacture of stainless steel and more particularly to improved stanles st el alloys and their treatment. An important aim of the invention is to provide superior mechanical properties, including exceptionally high strength as well as excellent hardness and other desirablecharacteristics in pars or articles ultimately fabricated from the alloys, a further aim being the attainment of these results while providing for production of the metal in a readily fabricable state prior to a convenient, final treatment wh ch produces the special mechanical properties. A fimher advantage of the new alloys is that as ultimately fabricated they also have a high degree of corrosion resistance, which is, of course, a property of essential importance in stainless steels. I I

In essence, these aims are achieved by the prov Isfon of a novel alloy which will allow not only the original forming of the metal to a desired shape, e.g. as by rolling to strip, sheet or the like, but which will also provide the desired fabricability, conveniently after afinil or so-called trigger anneal, to make various parts or articles, and which will thereafter be susceptible of a relatively simple treatment, that may specificaly include a refrigeration step, to transform the fzbricated structures or parts into ametallic state having the desired strength, hardness or the like. v s p Statedin another way, a .primary purpose of the invention is to produce an alloy w hich in one. condition isiductile or otherwise readily fabr'icable, preferably at temperature, and whichcan easily be transformed foanother condition where it has the special mechanical properties described above. Mo re specifically, the invs ti'on provides an improved alloy of the type that is first austenitic and thus easily formable, and that is readily processed to final hardened form by transformation to martensite and subsequent aging or tempering. Theultimate, hardened article of the novel alloy has properties, e.g. of tensile strength or yield strength, substantially superior to those heretofore obtainable in stainless" steels by any similarly expeditious treatmentorwithout using components that are more costly or that may have undesirable effects inother ways. The new alloy is particularly advantageous in that it does not require acontent of highly oxidizable elements such as aluminum an m- Theimproved stainless steelspf the invention attain the stated results by a novel combination of elements, in} ing certain special components, new in use or pro portion se um; coacting with specific base components release amt proportioned to have desired fundamental 2 characteristics as des'cribed above. The improvementin properties of articles or parts madefrom the alloys pear to be obtained, atleast in part, by the change. I phenomenon that is commonly called precipitationhard ing, and for commercial identification the co, pen I may therefore be referred to as new precipitatioiieharden ing alloys. It should be understood, howevencth'atthe invention is not limited or affected by the scientificacf; curacy or inaccuracy-of the term precipitation hardening or by any factual determination whether, the enhance;- ment of mechanical properties in the new alloys is o" is not accomplished by the same physical or chemical mechanism as has previously been described by that term. I v II As explained, the new alloy is hardened by, trans; formation to martensite, e.g. byrefrigeration. ,Foree fective development of the improvedproperties, the martensitic transformation is followed by a heat treat; ment, e.g. at about 850 F., which may be called a tempering or an aging treatment. Presumably this final operation brings about or completes the sorcalled precipif tation hardening, and also tempers or strengthens the martensitic structure; but whatever the actual phenomena, the efiectof this step is a marked enhancement, of. the mechanical properties, and appears to result, at, least in considerable part, from the new addition or additions in the alloy. I v

The new composition. is, as stated, one which can be: conditioned so that it will transform to r'nartensite at very low, e.g. sub-zero, temperature. Thus for in' tial, remit, ing, as in rolling to sheet, the alloy may be annealed at 1900 F. and as so annealed will be stable in the austenitic form; it is then given a special or triggerari' neal at, say, 1700 F.,' whereupon it will still be 'ausi tenitic down to room temperature, but is transformed to martensite on cooling to 30 to l00 F.. It will beu nderstood that if desired, other waysmaybe for...

lowed for effecting transformation of the inetal to: martensite or for conditioning the metalffor such tran formation, i.e. referring to the metal in its stable an tenite form existing after anneal at 1900? F. one such method is s o-called intergranular'.precipitation df agar: bori on holding the alloy at 1 4OQ".'Fahandanothe '5 cold working, which has obvious limitationslin prac ce.

In most cases, the operations leading toandinvol transformation by refrigeration are greatly preferred'as of special convenience (e.g. since final fabrication-Gait beperformed at room temperature after thet'riggerane, neal) and as providing best realization of all improved properties. I v I I One example of a base alloy susceptible Qfllifl formation to. martensite by low temperature cool g h I the following composition: 17% chromium, 4.5%,n'ickel; 2% manganese, 0.12% carbon and O.5%. silicon,'witli' the balance iron except for minor incidental imp This alloy, when annealed one or more times at ,IQOQ'E. (e.g. for rolling) is esseutiallystable austenite, but,aft a special or trigger anneal at I700" F. becomeskeapab of low temperature transformation, while" remain ng austen'itic at room temperature so as to permit ready 3 tempering treatment at about 850 F. for one hour or so. The low temperature treatment is effective to transform the structure to virtually 100% martensite, with some increase in the strength and ductility of the martensitic structure by the subsequent tempering operatIon.

As indicated, the present invention embraces the discovery that in stainless steel alloys of the above character, markedly improved mechanical properties of the ultimate product (i.e. after the described treatments) are achieved by certain new additions to the composition, which at the same time are such as to avoid some difficulties or disadvantages-that have attended prior proposals for improving this type of alloy. The elements constituting the new additions, in special proportions or relationships as described below, are boron, copper and phosphorus.

Reference is first made to boron, which has not heretofore been used or proposed for precipitation-hardening or like purposes; when included in amounts ranging from 0.02% to 0.7% of the alloy (all percentages herein being given by weight), it contributes significantly to the me chanical properties, especially in promoting or effectuating the final hardening of the metal. Especially good results have been achieved by the inclusion of both boron and copper, the former in a range upwards of 0.01% and the latter very preferably within an economically advantageous range of about 0.75% to 1.75%; the alloy containing both of these elements is regarded as a specific and particularly important aspect of the invention, affording markedly superior properties.

While copper in larger amounts, usually upward of 3%, or at least in amount of 2% or more, has been used or proposed for some strengthening effect in certain prior stainless steel alloys, it now appears that the described conjoint inclusion of both boron and copper in the present compositions, affords improvement significantly greater than is obtainable with either element alone, or with larger quantities of either element. In combination with one or both of the other new additions, however, copper can be included, if desired, in somewhat larger quantities than the range mentioned above, the greater range being 0.5% to 3.0%. For instance, when copper is so included along with bo'ron, a considerable measure of the advantage which attends the specific combination of these elements described above, is realized, but unusual practical advantage appears to accrue by keeping the copper content of the combination with'n the new, low range. Indeed, the addition of copper alone, in this special range (e.g. up to 1.75%), has a definite value for the strength of the ultimate Product, a result not heretofore appreciated or recognized with respect to the base alloys of the specific refrigerationhardenable type here contemplated.

A third element of the described group of new additions is phosphorus, incorporated in the alloy in amounts ranging from 0.08% to 0.40%; in such quantities, phosphorus also materially improves the metal, apparently in contributing characteristics of so-called precipitation hardening. Indeed although phosphorus, generally in larger amounts, has been employed for some benefical effect in other types of stainless steels, notably those characterized only by, or designed to remain in, an austenitic condition, it does not appear to have been heretofore known or apparent that phosphorus would or could be effective for precipitation hardening from martensite. Phosphorus may be used very effectively in combination with one or both of the other additions, for example in an alloy which also contains boron and copper, such as with the presently preferred proportions of boron, 0.1% to 0.3%, and copper, 0.75% to 1.5%. The content of phosphorus which is now preferred in combination with one or both of the other elements herein contemplated for effectiveness in hardening the martensitic form of the metal, is from 0.1% to 0.3%. As stated, however, the invention contemplates alloys in which phosphorus alone, from the described group, is employed for its newly discovered effect, i.e. up to 0.4%.

In presently preferred form, the alloy also includes molybdenum, as in amount ranging up to 4%, or preferably from 1.5% to 3.5%, with special benefit to the final, hardened products, notably in improving the strength of the material at elevated temperatures. Additional or alternative components are metals of the class consisting of tungsten and vanadium; although present evidence is that molybdenum is considerably more effective for such purposes, these other additions appear useful for enhancing some properties, viz. tungsten for high temperature strength and vanadium for improving the adaptability of the metal to drawing at higher temperatures, i.e. prior to transformation and hardening.

A composition of specifically superior properties, to which the invention in a correspondingly specific sense is directed, is an alloy of the described base type containing boron from 0.01% to 0.3%, copper from 0.75% to 1.75% and molybdenum from 0.15 to 3.5% especially preferred results being obtainable with boron in the more limited range of 0.04% to 0.2% and molybdenum between 2% and 3%.

As stated above, it appears that each and all of the new additions, viz. boron, copper and phosphorus in the proportions and relationships indicated, exert their improved effects primarily in connection with the final treatment, i.e. in the final aging or tempering after transformation to martensite has been effected. It is also of special significance that all of these and other additions (notably molybdenum) described above have no deleterious effect on other characteristics of the metal, and in particular do not adversely affect the readily fabricable character of the alloy in its annealed austenitic conditions, i.e. either after the working anneal at 1900" F. or the trigger anneal at 1700 F.

The special nature of the base alloy is critical for the ultimate results described, including the novel effects of the additions. Thus one important feature of the alloy of the invention is its essentially stable austenitic character after annealing at 1900 F., whereby it can be rolled or otherwise worked with relative ease and whereby it does not transform to martensite, even at temperatures far below freezing. A second important feature is the adaptability of the base alloy to assume less stable austenitic properties, for example the conversion effected by trigger anneal at 1700 F., whereby the metal remains austenitic and fabricable even at room temperature, yet it is easily and substantially transformed to martensite by refrigeration. After the transformation, the desired characteristics are further enhanced by the moderate heat treatment, i.e. so-called tempering. This combination of properties is achieved by appropriate balance, in the base alloy, of those elements which tend to stabilize ferrite (mainly chromium and silicon) and those ele ments which tend to stabilize austenite (e.g. carbon, nickel and manganese). Silicon is not essential of itself, but as a rule is unavoidably present and must therefore be taken into account in balancing the components of the alloy, as will be understood by those familiar with compositions of this type.

Furthermore, in processing, the extent of carbon taken into and remaining in solution is important, e.g. in that for refrigeration hardening there must be some carbon so remaining after the trigger anneal. For example, in one prior type of stainless steel alloy, there have been included elements such as titanium and columbium which tend to form carbides, substantially more strongly than chromium. Thus when such alloy, containing say up to 0.1% or 0.12% carbon, is annealed, for example at 1900 F., the strong carbide formers combine with the carbon, removing all but a very small amount (e.g. 0.02%) of the carbon from solid solution. It is understood that chromium itself does not remove carbon from solid solution '5 "at 1900" F. but as stated, the stronger carbide formers do so; In consequence, the alloy containing them, while austenitic at the annealing temperature, is largely depleted of carbon and therefore transforms or tends to transform to martensite upon cooling to room temperature. Martensitic material, of course, is not readily fabricable. v

In the base alloys to which the present invention is directed, the response to annealing treatments is consid erably different. If annealed at about 1900 F., these alloys, containing say 0.12% carbon, will thereafter retain all of such carbon in solid solution. With such carbon content, the structure is still austenitic upon cooling to room temperature or even below. Conversion to martensite of the material in such state is not readily attainable. If, however, the alloy is alternatively, or there'- after, annealed at about 1700 B, it appears that some ofthe carbon is precipitated as chromium carbide, for example about 0.05%, while the remainder, about 0.07%, stays in solid solution. It further appears that this reduction of the amount of carbon in solution adjusts the transformation characteristics of the austenite so that it will transform to martensite when cooled to a readily attainable point below room temperature, e.g. in the range of 30 F. to l F. At the same time, the material is sufliciently austenitic, following the trigger anneal at 1700" F., to be readily fabricated at room temperature, as by many types of operation such as cutting, punching, bending, pressing, drawing, stamping and machining; austenitic structures have desirable forming characteristics, being relatively soft and ductile. Hence an important attribute of the selection and proportion of the alloy components is that some of the carbon will remain in solution after the trigger anneal, whereby the stability of austenite is modified no further than is necessary to provide martensite transformation at a convenient, sub-zero temperature.

It may also be noted that some variation in the content of carbon in solution after the final anneal can be availed of toadjust the processing and ultimate characteristics of the alloy. That is to say, increasing the dissolved carbon content lowers the temperature of martensite formation, but also increases the strength of the martensite formed.

Moreover, the alloy is such, as previously explained, that transformation to martensite can be reached in other Ways. For instance, if it is finally annealed at 1400 F. instead of 1700 F., the stability of austenite is more considerably reduced (presumably by greater removal of carbon from solution), so that the metal will become martensitic when or even before room temperature is reached. Alternatively, cold working, if suitably uniform, can have a like effect, indeed of immediately causing transformation to martensite.

A considerable number of tests have demonstrated the new results'of the alloys of the invention; representative groups of such tests are reported below, such reports also serving to provide description of suitable examples of the new compositions.

In one set of tests a series of alloys were prepared, in all cases including a base composition of 17% chromium, 4.5% nickel, 2.0% manganese, 0.12% carbon and 0.50% silicon, with the balance iron (and incidental impurities) except for additions in accordance with the invention as stated-below. This base is identified in Tablel below as Base No. 1. Some alloys were also prepared with the same base composition except that manganese was present in amount of 2.5% instead of 2.0%, these alloys being described as including Base No. 2. As willbe seen from the table, the alloys specifically prepared included Base No; 1 without additions, and various compositions of the invention, consisting of Bases No. 1 and No. 2 respectively with further elements. In each case, a test ingot of the specific alloy was cast, and appropriate specimens of size and shape to suit the standard requirements for the several tests were prepared. The operation on each test s'p'ecimen included annealing the specimen at 1700 F.

F. for eight hours. After this chilling or freezing treatmerit, the specim'en w'asiallowed to resume room temperattire and was thereafter treated by heating to 350 F: for one hour. 7 All tests in the table were made with specimeris which had been subjected to this. series of opera tions. U M

The properties tested were hardness, yield strength, ten

sile strength and elongation, the results for the vari alloys being as follows: a 5

TABLE I 0.27 1 Elongat 9n, Rockwell Oflst Tensile Percent Alloy Yleld- Strength,

Hardness Strength, 1,000 p.s.i. v 7

Q Base No.1 72 187 212 1425 Hid Base No. 1+0.30% B 73. 5 200 21s a 4.5 Base N0. 1+1.5% Cu 73. 5 201 222 14,5 8.5. Base No. 1+0.15% P 74. 5 209 257 9. 5 8 Base N0. 1+1.0% Cu 1 0.15 74.5 207 252 11.5 V 9.5

75 2'22 240 5.5 5.5" 75.5 251 245 6. 5" i 74.5 212 237 10.5 .a 74 217 235 9.5 7.5 Base No. 2+0.5% Cu 73.5 202 a 224 17.5 13 Base Na.2+O.15%P 75.5 194 223 19 15 It will be seen that all of the alloys containingthe 555 additions or combinations of them showed marked i provement of physical properties, particularly hardn (over 73), yield strength (improved by 5 %'20%)' and tensile strength (increased, from several percent to 15 (26'), as distinguished from the base alloy, exemplified by'ba a" alloy No. 1. Tests of all the characteristics were also made of specimens of each of the above tabulated alloys, where the test pieces were simply annealed to 1 700" E for one-half hour and then cooled to room temperatu" without the subsequent treatments of freezing andh'e ing. In such cases, the Rockwell A hardness' rang d from 54.5 to 62, the offset yield strength from 42000 t" 57,000 p.s.i. and the tensile strengthfrom' 158,000, ,t o 192,000 p.s.i., with elongations ,in both one'inch and w inches ranging from about 13% to about 25%. In g eral, the variations among the alloys in the' 'trigger nealed but otherwise untreated condition seem to Y, no special relation to the alloying additions. In all ca such properties were representative of the aus'tenitiecom dition and indicated ready adaptability of the metalto boron and copper, i.e. in amounts respectively ranging I upward from 0.01% and 0.5% and within the upp limits indicated hereinabove, exceptionally good mecham cal properties are achieved. Although the'various addi tions did not in general reveal any deleterious effecton the base alloy inits' annealed condition, i.e. relative' to fabricability, there was some indication that boron alone" 5 tended to reduce ductility to some" extent, whereas-if boron and copper were included together, suchreductioii of ductility (relative to the base alloy) was not apparent The following table shows high temperature test rq-"r sults of a number of further examples of alloys; contain; ing the copper-boron addition, and also other compo":

nents, viz. molybdenum as now preferred for perature properties, or alternatively tungsten, or both molybdenum and vanadium:

0.08% phosphorus. It is noted that the stated minimum amount of phosphorus (i.e. minimum for effectiveness) TABLE H Composition Tensile Alloy Strength Number at 800" F Mn Si Ni Cr Cu B Mo W V 1,000 p s 1 0. l2 2. 5 0. 5O 4. 5 16. 5 1. 0 0. 20 176 0. 12 2. 5 0.50 4. 5 16. 0 1.0 0. 20 198 0. 12 2. 5 0.50 4. 5 15. 5 1. 0 0.20 195 0.12 2. 5 0.50 4. 5 16. 5 1.0 0. 20 195 0.12 3. 0 0.50 4. 5 15. 5 1. 0 0. 20 203 0. 127 2. 06 0. 44 4. 45 16. 50 0. 91 0. 10 172 0. 125 2. 0S 0. 44 4. 35 16. 50 0. 92 0. 10 182 Alloys containing one or more of the ingredients molybdenum, tungsten and vanadium appear to have unusual practical value, the inclusion of molybdenum being of special significance in making a composition which is exceptionally useful for the purposes here contemplated.

While it will be understood that in general the desired mechanical properties of the alloys are determined by the selection and mutual proportions of the various components and by appropriate balance of their effects, as is indicated above and in some respects more fully below, and likewise that the total of alloying ingredients or of a major group of them is not alone a determinant of superior properties, it may be mentioned that preferred examples of the composition have usually contained from about 23.5% to about 29.5% of elements other than iron, and mostly from about 25% to about 28% of such elements. Like experience is that the total of elements selected from the group of chromium, molybdenum, silicon, nickel and manganese has been from about 22.5% to about 25.5% in the preferred alloys, and more specifically about 24% to about 25.5% in the best examples tested.

As indicated, although at present there is reason to describe the additions of boron, copper and phosphorus as functioning by causing so-called precipitation-hardening, it is possible that these elements act in some other way, either wholly or in part, to improve the transformation properties, e.g. in the general course of conversion to martensite by refrigeration and in further change during the subsequent heat treatment. It seems clear, however, that these elements do not function as carbide formers; indeed, on the contrary, they do not interfere with the desired maintenance of a significant proportion of the carbon in solution following certain annealing stages, so as to keep the metal substantially as austenite at room temperature.

Some variation in the proportions of ingredients may be made, within the requirement that the base alloy involve appropriate balance of elements stabilizing ferrite and elements stabilizing austenite, i.e. balance of such elements to achieve the desired austenitic character of the annealed metal and subsequent hardening by transformation, in a manner which will now be readily understood. Thus in its broadest ranges the base alloy composition (all proportions here, as elsewhere, by weight) may consist essentially of 12% to chromium, 1% to 7% nickel, 0.5% to 5% manganese, 0% to 1% silicon and 0.06% to 0.2% carbon, the balance in all cases, except for the special additions and except for incidental impurities to be expected in these alloys, being iron. A

further critical limitation, however, is that the total content of nickel and manganese (which tend to stabilize austenite) be no higher than 7.5%, i.e. in order to assure the desired capability of transformation. The alloy can be defined as including (with the above components) the special additions as: 0% to 0.7% boron, 0% to 3.0% copper, and 0% to 0.4% phosphorus, at least one of these elements being present in amounts of: at least 0.04% boron, 0.75% to 1.75% copper, and at least is significantly greater than quantities of this element which may be adventitiously present. Useful effect of boron, moreover, in promoting precipitation-hardening or the like, has been noted for amounts of this element down to 0.01%, quantities of 0.02% or more being somewhat preferred. In addition, the alloy may embrace, and its description therefore includes: 0% to 4% molybdenum, 0% to 4% tungsten and 0% to 1% vanadium.- Among incidental impurities mentioned above, there may be, for instance, sulfur up to a maximum of 0.04%.

In a specific sense the advantages of the invention are realized in a definitely superior way, by base alloys where the above-named ingredients for such alloy are present in about the following proportions (now greatly preferred): 14.5% to 17.5% chromium, 3% to 5.5% nickel, 2% to 4% manganese, 0.3% to 0.7% silicon and 0.10% to 0.14% carbon, it being understood that the same upper limit for total nickel and manganese prevails, and that silicon may be omitted but is almost invariably present, to at least the stated minimum, as a necessary consequence of alloying and melting procedures. Other components of the preferred alloys are 1.5% to 3.5% molybdenum, 0% to 1.5% tungsten and 0% to 0.4% vanadium. The addition of the new components can be included as indicated above, a preferred maximum for phosphorus being 0.3%, and a preferred range for boron being 0.04% to 0.30% (with special preference for some purposes at 0.10% and above) and for copper 0.75% to 1.5%. The balance of the alloy is, as previously stated, iron, except for minor incidentals.

it will be readily understood that the components of the base alloy can be mutually proportioned, under principles known in the art, for provision of optimum balance between elements tending to produce austenite and elements tending to produce ferrite at elevated temperatures, and also for such total of the basic alloying ingredients as will afford the optimum desired character of austenite stability. Account is preferably taken, in such balancing, of additional components of the nature of molybdenum and the like; thus when molybdenum is included, less chromium may be used. For example, the chromium content can be reduced, from 17% or thereabouts, by 0.7 (percentage as based on the total alloy) for every 1% of molybdenum added; on the other hand it may sometimes be desirable to keep a relatively high level of chromium when molybdenum is present (Table III below), e.g. thus permitting a small sacrifice of mechanical properties in favor of retaining the highest corrosion resistance.

Another and particularly useful example of a specific composition of the alloy, expressed as a close range of percentages for each element or an approximate percentage (to account for tolerance in commercial production) is: 15% to 17% chromium, 4% to 6% nickel, 2% to 2.5% manganese, 0.3% to 1% silicon, 0.10% to 0.15% carbon, 0.5% to 1.0% copper, 0.10% to 0.20% boron, 2% to 3% molybdenum and 0.10% to 0.38% vanadium, with the balance iron (maximum sulfur 0.025%). Thus a specific heat of stainless steel made to conform with seem the nominal composition just outlined, had" an analysis as follows:

TABLE III Percent Chromium 16.24 Nickel 4.25 Manganese 2.21 Silicon 0.43 Carbon 0.123 Copper 0.9 Boron 0.14 Phosphorus 0.014 Molybdenum 2.20 Vanadium 0.38 Sulfur 0.010 Iron a Balance A further example of the new alloy was a relatively large heat having the following composition, as analyzed:

Metal from this heat was cold rolled, with annealing at 1950" F. It was subsequently re-annealed at 1700" F. (being the trigger anneal), and pieces were thereafter refrigerated at 100 F. for one hour and finally aged for six hours at 875 F. The resulting metal showed a tensile strength of 205,000 p.s.i., 0.2% offset yield strength of 185,000 p.s.i., and elongation (in 2 inches) of 7%.

In making the alloys, conventional procedures can be followed, i.e. as employed heretofore for the production of stainless steel alloys. be produced in an electric furnace or other suitable equipment, with the alloying ingredients all there included, or alternatively by including most of such ingredients in the furnace, with ladle additions of special ingredients present in minor amounts. The form of adding the various elements may be such as is customary for incorporating these elements in other iron and steel compositions, for example as by the use of various ferro alloys where appropriate.

The produced metal may be cast in conventional ingot form, and the ingots thereupon reduced, for example by hot rolling or by combinations of hot rolling or forging, and cold rolling, to appropriate stock, viz. sheet, strip, bar, rod or the like. Where cold rolling is used, intermediate annealing may be effected at 1900 F. or thereabouts; with this anneal, which has been described hereinabove and which is called a solution anneal, a completely austenitic form is thus maintained, as best suited for mechanical deformation. The metal can be fabricated into the desired part, blank or other article at this time, and if it is to be hardened by refrigeration, it can thereafter be subjected to the trigger anneal at 1700" F. Alter.- natively, such fabrication can be postponed until after the trigger anneal. In either case, hardening is then eifectuated by refrigeration and subsequent aging or tempering.

Described more generally, solution annealing (which is employed regardless of the selected mode of transformation to martensite) is effected by treating in the range of about 1850 F. to about 2000 F., preferably about 1900 F., and cooling rapidly. For ultimate refrigeration hardening, the trigger anneal preferably comprises'treat ing at about 1700 F. to 1800 F., and cooling rapidly; Annealing time is not critical for either step; for example, a time of ten minutes to one-half hour or so is suitable for the trigger anneal. If the metal has been trigger annealed prior to ultimate fabrication, it need not be so treated afterwards unless it was welded or nonuniformly deformed. Stated broadly, the range for martensite transformation at sub-zero temperatures is approximately -30 F. to -'-120 F.; the mechanism may in fact involve some beginning of transformation as the metal is cooled below freezing, with increase as the temperature is brought below 0 F. Particularly effective results, especially for assurance of complete transformation, are achieved by holding the metal in the range of 80 F. to 120 F., for a minimum of two hours, one preferred treatment being to hold it at 100" F. for eight hours.

If the alternative procedure of conversion to martensite at room or higher temperature is used, the solution-annealed metal is subjected to a special anneal at about 1400 P. On cooling, the transformation occurs as or before room temperature is reached. Thus in a broad sense, the range of trigger annealing may be said to ex tend from about 1350 F. to 1850 F. At the upper end of the range, the stability of austenite is only modified to the point where it will transform at very lower temperatures, whereas at the lower end of the range, the

. stability is greatly reduced (more carbon being removed That is to say, the melt may from solution), whereby the transformation temperature or range is elevated, even to about 200 F. In general, distinctly better mechanical properties are obtained with the procedure which requires refrigeration for transformation.

Another alternative treatment involves cold working or other straining, for example as by so-called stretch forming. In general, this mode of treatment can only be relied upon for transformation effects if it is uniform throughout the piece of metal. Straining, i.e. as effected on metal that has previously been solution-annealed, can be effective to reduce the stability of austenite (e.g. as a substitute for trigger anneal at 1700 F), or if of substantially greater magnitude, can bring about the transformation to martensite. For the first of these effects, a minimum of 10% to 15% elongation, throughout the work, is required; an alloy of the present type, so treated, will: transform by refrigeration, without any trigger an"- neal. If there is substantially greater strain, as by more cold rolling or stretch forming, the transformation is brought about by such operation itself, i.e. without any low temperature treatment. For instance, reduction of 30% to 60% by cold rolling (preferably a reduction toward the upper end of such range), is generally needed in order to accomplish the martensite transformation by straining alone. It will be understood, of course, that the stated percentages of reduction by rollingwill usually involve substantially lower percentages of actual elongation.

In all cases the metal is given the final aging or tempering treatment, preferably by holding in the range of about 800 F. to about 1100 F. for a minimum of one hour. The time may be considerably longer; indeed the treatment may be varied to adjust final properties. For instance, higher tempering temperatures, within the range, result in a property of higher elongation, but lower strength. Treatment at 850 F. for one hour or more is found to produce a good combination of strength and ductility.

By way of example, the following are instances of suitable forming, processing and fabricating operations for alloys of the present invention. In these examples, it may be assumed that the metal is first hot rolled (or otherwise formed, as by cold rolling) to strip, and then solution annealed at 1900 F. for ten minutes, and air 1 1 cooled; each procedure thus deals with solution-annealed strip.

Procedure A (1) Fabricate (2) Trigger anneal at 1700" F. for 10 minutes and air cool (3) Refrigerate at 100 F. for 8 hours (4) Temper at 850 F. for one hour and air cool Procedure B (1) Cold roll to 30% reduction or more'(e.g. 30% to (2) Temper at 850 F. for one hour (3) Fabricate (fabrication limited in this instance) Procedure C (1) Fabricate (2) Anneal at 1400" F. for 2 hours and air cool (3) Temper at 850 F. for one hour and air cool It will be seen that not only the alloy but also its mode of treatment, can be considerably varied to suit many different requirements or conditions of use. In all cases, there is provision for convenient working and fabrication, and for eventual treatment to reach desired, very high characteristics of strength, hardness and the like.

It is to be understood that the invention is not limited to the specific examples and operations herein described, but may be carried out in other ways without departure from its spirit.

We claim:

1. A stainless steel alloy consisting essentially of about: 12% to 20% chromium, 1% to 7% nickel, 0.5% to 5% maganese, 0.06% to 0.2% carbon, not more than 1% silicon, the total of nickel and manganese being not more than 7.5%; not more than 4% molybdenum, not more than 4% tungsten, not more than 1% vanadium, not more than 0.4% phosphorus, 0.75% to 1.5% copper, 0.04% to 0.3% boron, and the balance iron.

2. A stainless steel alloy consisting essentially of about: 12% to 20% chromium, 1% to 7% nickel, 0.5% to 5% manganese, 0.06% to 0.2% carbon, and up to 1% silicon, the total of nickel and manganese being not more than 7.5%, not more than 4% molybdenum, not more than 4% tungsten, not more than 1% vanadium, not more than 0.4% phosphorus, 0.5 to 3% copper, 0.01% to 0.7% boron, and the balance iron; the content of ingredients as aforesaid in the alloy characterizing it as one which is austenitic upon annealing at a temperature in the approximate range of 1850" F. to 2000 E, which is transformable to martensite from said austenitic form and which in the martensitic form is adapted to be improved in mechanical property by treatment at a temperature in the approximate range of 800 F. to 1100 F.

3. A stainless steel alloy as defined in claim 2, which contains not more than 1.5% tungsten, not more than 0.4% vanadium, and 0.04% to 0.3% boron.

4. A stainless steel alloy consisting essentially of about: 14.5% to 17.5% chromium, 3% to 5.5% nickel, 2% to 4% manganese, 0.10% to 0.14% carbon, and up to 0.7% silicon, the total of nickel and manganese being not more than 7.5%; 1.5% to 3.5% molybdenum, not more than 1.5% tungsten, not more than 0.4% vanadium, not more than 0.3% phosphorus, 0.75% to 1.5 copper, 0.01%"to 0.3% boron, and the balance iron.

5. A stainless steel alloy consisting essentially of about: 14.5% to 17.5% chromium, 3% to 5.5% nickel, 2% to 4% manganese, 0.10% to 0.14% carbon, and up to 0.7% silicon, the total of nickel and manganese being not more than 7.5%; 1.5% to 3.5% molybdenum, not more than 0.4% vanadium, 0.75% to 1.5% copper, 0.02% to 0.3% boron, and the balance iron.

6. A stainless steel alloy consisting essentially of about: 15% to 17% chromium, 4% to 6% nickel, 2% to 2.5% manganese, 0.1% to 0.15% carbon, 0.3% to 1% silicon, the total of nickel and manganese being not more than 7.5%, 2% to 3% molybdenum, not more than 0.4% vanadium, 0.5% to 1.5% copper, 0.02% to 0.2% boron, and the balance iron; the content of ingredients as aforesaid in the alloy characterizing it as one which is austenitic upon annealing at a temperature in the approximate range of 1850 F. to 2000 F., which is transformable to martensite from said austenitic form and which in the martensitic form is adapted to be improved in mechanical property by treatment at a temperature in the approximate range of 800 F. to 1100 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,144,713 Becket Jan. 24, 1939 2,432,616 Franks et a1 Dec. 16, 1947 2,482,097 Clarke Sept. 20, 1949 2,624,670 Binder et al. Ian. 6, 1953 2,797,993 Tanczyn July 2, 1957 2,799,577 Schempp et al. July 16, 1957 2,815,280 Clarke Dec. 3, 1957 2,826,496 Kegirise Mar. 11, 1958 FOREIGN PATENTS 375,793 Great Britain June 20, 1932 

1. A STAINLESS STEEL ALLOY CONSISTING ESSENTIALLY OF ABOUT: 12% TO 20% CHROMIUM, 1% TO 7% NICKEL, 0.5% TO 5% MANGANESE, 0.06% TO 0.2% CARBON, NOT MORE THAN 1% SILICON, THE TOTAL OF NICKEL AND MANGANESE BEING NOT MORE THAN 7.5%, NOT MORE THAN 4% MOLYBDENUM, NOT MORE THAN 4% TUNGSTEN, NOT MORE THAN 1% VANADIUM, NOT MORE THAN 0.4% PHOSPHORUS, 0.75% TO 1.5% COPPER, 0.04% TO 0.3% BORON, AND THE BALANCE IRON. 